1. Field of the Invention
The present invention relates to operational amplifiers. More particularly, it relates to compact transconductance (g.sub.m) control circuits for CMOS rail-to-rail input stages operating in strong inversion.
2. The Prior Art
Input stages of low-voltage amplifiers often have to be able to deal with rail-to-rail common-mode input voltages. To obtain a rail-to-rail common-mode input voltage swing, a P-type and N-type input pair can be placed in parallel.
The common mode input range of such a rail-to-rail input stage can be divided in to three parts: (1) Low common-mode input voltages (only the P-channel input pair operates), (2) Intermediate common-mode input voltages (both the P-channel and N-channel input pairs operate), and (3) High common-mode input voltages (only the N-channel input pair operates).
When the common-mode input voltage moves from one part of the common-mode input range into another, the transconductance (g.sub.m) changes with a factor of 2. Since the unity-gain frequency of an amplifier is proportional to the unity-gain frequency of the input stage, this impedes optimal frequency compensation. In order to obtain the maximum unity-gain frequency over the whole common-mode input range, the g.sub.m of the input stage has to be constant.
U.S. Pat. No. 4,555,673 to Huijsing et al., discloses a differential amplifier with rail-to-rail input capability and controlled transconductance (g.sub.m). The method employed for controlling the transconductance uses current control, or current switches, to steer at least part of the supply current away from at least one of the differential portions of the input stage when the common mode voltage is in at least one part of the supply range. As a MOS transistor operating under the weak inversion regime behaves like a bipolar transistor, the same techniques can be used for a MOS transistor operating in weak inversion.
In weak inversion, a constant-g.sub.m can also be obtained by using a minmax circuit (Botma, J. H., R. F. Wassenaar and R. J. Wiegerink, "Simple rail-to-rail constant-transconductance CMOS input stage in weak inversion", Electronic Letters, No. 12, pp 1145-1146, June 1993). However, in many applications, such as high-slew rate or high bandwidth, the g.sub.m of an input transistor operating in weak inversion is too small. If the input pairs operate in strong inversion, its g.sub.m can be larger.
Existing constant g.sub.m -control of an input stage operating in strong inversion use square root circuits (Botma, J. H., et al., "A low-voltage CMOS operational amplifier with rail-to-rail constant-gm input stage and class-AB rail-to-rail output stage", Proceeding ISCAS93, pp.1314-1317), or three times current mirrors (Hogervorst, R. et al., "CMOS low-voltage operational amplifiers with constant-gm rail-to-rail input stage", Proceedings ISCAS92, pp.2876-2879). The g.sub.m -control with three-times current mirrors can form a positive feedback loop at very low supply voltages. This effect can be avoided by using an additional protection circuit. (R. Hogervorst, J. P. Tero, R. G. H. Eschauzier, J. H. Huijsing, "A compact power-efficient rail-to-rail input/output amplifier for VLSI cell libraries". in Digest ISSCC94, February 1994).
The minmax g.sub.m -control circuit, which is used in rail-to-rail input stages operating in weak inversion, can easily be adapted to input stages operating in strong inversion, by choosing the width (W) to length (L) ratio of the g.sub.m -control transistors 3-times larger than the input transistors.
These existing gm-control techniques have their own drawbacks. Some are designed for weak inversion and therefore are not appropriate for strong inversion. In strong inversion, three-times current mirrors, minmax circuits or square-root circuits can be used. Three-times current mirrors and minmax circuits still display a 15% variation over the common-mode input range. In addition, the g.sub.m -control with three times current mirrors can form a positive feedback loop at low-supply voltages. Square root circuits display lower variation of the g.sub.m, however, these types of circuits tend to be complex, and as such, occupy a large amount of die area.
Therefore, it is desirable to have a transconductance (g.sub.m) control circuit for rail-to-rail input stages operating in strong inversion that is compact in design, and maintains the g.sub.m of the input stage constant over the whole common-mode input range. The more compact the design, the less die area that will be required. It is also desirable to have a control circuit which is robust against parameter variations, such as supply voltage variations.